The present invention relates to a laser beam printer, digital copier, facsimile apparatus using plain papers or similar image forming apparatus and more particularly to a belt type fixing device included in an image forming apparatus.
An image forming apparatus includes a fixing device for fixing a toner image formed on a paper or similar recording medium. While the fixing device has traditionally been implemented by a heat roller, a film heating type fixing device, a belt type fixing device, other fixing devices including an electromagnetic induction type fixing device and a resistance heating type fixing device are available today.
The fixing device of the type using a heat roller is feasible for a full-color image formed on a paper by much toner and requiring the surface of the paper to be glossy. The heat roller is made up of a hollow metallic core, an elastic member covering the core, and a surface layer covering the elastic member and having a high parting ability. A heat source is disposed in the core and implemented by a halogen heater. However, a substantial warm-up time is necessary for the surface of the heat roller to be heated from the initial environment temperature to the expected heating temperature. This is because the core has a great thermal capacity and because a substantial period of time is necessary for heat to reach the surface layer via the elastic member. The fixing device lacks a quick starting ability.
On the other hand, a belt type fixing device is usually applied to a laser beam printer, digital copier, facsimile apparatus using plain papers or similar image forming apparatus. This type of fixing device is applicable even to other apparatuses as a device for heating a paper while conveying it. Japanese Patent Laid-Open Publication No. 9-212031, for example, discloses a fixing device including an endless belt passed over a plurality of conveyor rollers, and a roller contacting the outer surface of the belt for applying tension to the belt or cleaning the belt. The belt has a smaller thermal capacity than the heat roller and therefore reduces the warmup time of the fixing device. There is, however, an increasing demand for an even shorter warm-up time.
Assume that a roller is held in contact with the outer surface of the belt for applying tension to the belt or cleaning the belt, as stated earlier. The problem with this configuration is that offset toner deposits on the belt and adheres the belt and roller. Such toner is driven at temperature lower than its softening point because a heat source is remote from the roller, scratching or otherwise damaging the belt.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 9-138597 and 10-78725.
It is therefore an object of the present invention to provide a belt type fixing device capable of reducing the warm-up time and preventing a belt included therein from being adhered to a roller by toner and damaged.
It is another object of the present invention to provide a belt type fixing device capable of removing offset toner, paper dust and other impurities from the surface layer of a belt included therein.
It is another object of the present invention to provide a belt type fixing device capable of surely and efficiently cleaning a belt included therein.
It is another object of the present invention to provide a belt type fixing device including a hollow roller having a surface layer whose coefficient of friction is small enough to protect a belt included in the device from damage.
It is another object of the present invention to provide a belt type fixing device capable of increasing yield and reducing cost.
It is another object of the present invention to provide a belt type fixing device including a hollow roller whose surface temperature can be accurately controlled.
It is another object of the present invention to provide a belt type fixing device capable of lowering the turn-on frequency of a second heat source included therein to thereby save power.
It is another object of the present invention to provide a belt type fixing device capable of reducing the warmup time and obviating defective images ascribable to the damage of a belt included therein.
In accordance with the present invention, a belt type fixing device includes an endless belt, a conveyor roller supporting the belt such that the belt is rotatable, a first heat source for heating the belt, a press roller for pressing a recording medium against the belt, a hollow roller contacting the outer surface of the belt, and a second heat source disposed in or adjoining the hollow roller. The hollow roller is rotatable in the same direction as the belt, as seen at a position where the former contacts the latter, at a peripheral speed equal to or higher than the peripheral speed of the belt.
Also, in accordance with the present invention, a belt type fixing device includes an endless belt, a conveyor roller supporting the belt such that the belt is rotatable, a first heat source for heating the belt, a press roller for pressing a recording medium against the belt, a hollow roller contacting the outer surface of the belt, a second heat source disposed in or adjoining the hollow roller, and a temperature sensor for sensing the temperature of the surface layer included in the hollow roller. The second heat source is controlled on the basis of the temperature being sensed by the temperature sensor.
Further, in accordance with the present invention, in an image forming apparatus including a belt type fixing device, the belt type fixing device includes an endless belt, a conveyor roller supporting the belt such that the belt is rotatable, a first heat source for heating the belt, a press roller for pressing a recording medium against the belt, a hollow roller contacting the outer surface of the belt, and a second heat source disposed in or adjoining the hollow roller.